Friday, 24 March 2017

Thermostats (again)

(Update: Sorry to my wife. According to all of my five kids I am being tactless, again, which I know I can be. But some interesting comments have been made on this post. Perhaps the best is don't argue with the cook!)

Blazing row with my wife, again. This time over gas oven.

She is adamant that you start by setting the oven to max (gas mark 9) first and then down to what you want, (e.g. gas mark 6) to get it to temperature quicker, and apparently the man that came to repair the oven today said the same.

I am sorry, but the gas oven, like most devices using a thermostat, has a binary output (on or off). It is either a low level pilot or full blast. That is it. I am sure some systems work differently, but most thermostats are binary output, as that works, and is simple.

They have a target temperature, a current temperature, some level of internal hysteresis, and they have a binary output. Pretty much end of story. Fact.

Of course, being a bit scientific, I went and checked. I would not want to be stating fact when I am wrong. I turned oven on to gas mark 1, door open. There is an initial low temperature state where output is in "low" pilot light, but after a few seconds it goes to the expected full blast. Changing dial to gas mark 9 does not change level of blast. There clearly are only the two options, pilot/low and full blast. That is it, as expected.

This means even if you only want gas mark 1, selecting gas mark 1 gets to temperature just as fast, at full blast, as selecting gas mark 9 to start with. There simply is no point in this ritual. Indeed, it can lead to the oven being over temperature and having to cool down. I think, for a cook, that is also bad. I am not a cook.

This simple fact about the way thermostats work in so many systems, whether the car air-con, the house air-con, the cooker, whatever, is apparently beyond my wife's comprehension. I don't know why, she is not stupid.

She is apparently sick of me lecturing her! Well, duh, if she finally got this really simple fact of life I would stop doing so. That is sort of how lecturing people when they are wrong on something works! You keep doing it until it sinks in. Well, I think that is the rule. It is so frustrating. As I tried to explain, black is black and white is white and if she was insisting that was not the case I would lecture her until she accepted it. This is not different. It is a simple fact of life, not opinion, not something that is unknown, is is so simple.

Why is this hard?

Update:

First off, yes, absolutely, I may be wrong in some (or all) aspects of my understanding of how this thermostat works in this case. Having played around with the oven a bit (not done proper tests with thermocouples and the smart meter measuring gas flow yet) I think I am not, but I may be. Even so, that does not mean what she is saying is right. However, if I am wrong, I am prepared to listen to reasoned arguments and references, and change my view and learn something. I'll even apologise.

What makes it so frustrating is that I have something to back my point of view - I can explain how a thermostat works, and why there is a binary output, and so on. The other side of the argument comes down to "stop lecturing me" rather than any explanation of why I might be wrong. I might indeed be wrong, but that is not the way to have any rational discussion, and that is what winds me up, sorry.

Why would I be wrong? Well, firstly, I don't like to say how something works without knowing how it works. I am sure that happens some times, when I have deduced how something works but got it wrong, but obviously I don't like to be wrong - who does? I am pretty sure I dismantled something with a thermostat in it when I was a kid - so I saw, first hand, how it works. In most cases it is the same today, I am sure, where the output is binary - on or off. This means in the case of an oven you do run at "full blast" until you get to temperature and then stop, and the cycle on/off in some way. Interestingly someone has suggested that this may not be the case, so I am hoping some time to test that. It was suggested there is a linear control near temperature. I can believe that, but it seems unnecessarily complicated and expensive. It is also suggested that the oven will not be fully at the right temperature when the thermostat in one point thinks it is. Even so, getting to temperature by setting a wrong and higher temperature and letting it then cool is likely to take longer over all, so my overall conclusion would be right. The idea of being hotter before opening the door or putting in a cold container is an interesting one, maybe that has some merit.

The additional heater on the thermostat is interesting. I have heard of this before, and I am pretty sure that it effectively reduces or cancels out the temperature hysteresis inherent in a sprung bi-metalic strip based thermostat, possibly even introducing a more controlled time based hysteresis. This would mean the switching between on and off at the target temperature is more rapid even if the temperature is stable thus creating an (albeit slow) pulse width modulation effect to maintain a proper temperature without the temperature swings you would expect with the simple temperature based hysteresis. This is clever, but won't change what happens in the time before you get to the selected temperature - which will be full blast all the way.

In light of the comments so far, I feel I am quite correct in saying that putting it up to 9 to get to temperature fasters is just silly, a meme, and old wives tale, and not needed. Even so, I may have something to learn about the details of how it does work, and will see if I can find the time to test and learn something.

P.P.S...

I am not alone :-

22 comments:

  1. Were this an electric oven you would be correct, but this a gas oven so maybe not. Repeat the experiment as follows - set a temperature and allow the oven to heat up. Now vary the set temperature slowly - you will probably find that there is a narrow range of temperatures over which the flame varies linearly, not just on or off.

    Gas thermostats are the usual bi-metallic things but they can drive the cut-off valve more than just on or off. If you are a long way under the desired temperature then the valve will be fully open and it won't matter in a cold oven whether you have it set to 180°C (mark 4), 220°C (mark 7) or 250°C (mark 9), as you observe. But once you get close to the temperature the flame should regulate down to the level needed to keep it steady.

    Is your wife right - probably; by setting it to mark 9 you will keep the valve fully open, and therefore the rate of temperature increase will be at its maximum, if you set it to the desired temperature it will start to close the valve off well before the target temperature is reached and slow the rate of heating. What the difference is you could only tell with a thermocouple inside the oven and some data logging, it probably won't be all that much though.

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    1. I may do more tests then, but the binary output of most thermostats is pretty standard.

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    2. Not arguing with that, in fact the oven thermostat will be /fairly/ binary but my guess is that you will find that there is a narrow linear region in which the oven will actually be operating when it is at, or near, the set temperature.

      I doubt that the original designer of these valves thought "it needs a linear region" but the valve will need to turn a bit between fully on and fully off and will need a bit of torque to applied to do so. The amount of torque available will depend on the temperature and the control position which just fiddles with the tension in the system. The result is, as I said, a system which does have a linear response around the set temp.

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    3. Interesting. As you say, thermostats is binary, but if the effect is more linear that is fun. Even so starting at 9 will not help speed it up even in that case unless you go over temperature and so add time for it to cool. It is an old wives tale, a meme, and deserves to be quashed.

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  2. Thermocouple and graph it over time. You are saying it works a particular way based on your model. You may be correct but if your model doesn't fit the data it's time for a better model.

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    1. Absolutely. This will take a while to set up, partly as my wife has to be out of the house :-) and if my model is wrong I will stand corrected and also apologise. That does happen from time to time, usually because technology has changed size "my day" which basically means since the time I learnt how something works (worked).

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    2. But a single thermocouple will only measure the temperature at one point. It won't measure the average oven temperature which, for cooking, is much more important than a spot temperature.
      To do it properly, you'd need multiple measurements - air, shelves and buried in the oven wall. Then you'd need to measure the temperature drop when you add a thermal mass.
      My theory is that an oven is useless if it doesn't store some heat - without some form of storage, the temperature drop when the door is opened and something is put in would be significant. The question, as I see it, is does the thermostat measure the average temperature of the whole oven (in which case, there's no benefit to putting the oven on high to warm up faster), or whether it only measures air temperature at a specific spot (in which case turning the oven up high will get it to the desired temperature faster).
      Personally, I'd just set it to the temperature I want and wait.

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  3. I am of the understanding though that you should before opening the door stick it up a notch, leave it for a moment, and then open the door as opening it cools it. Maybe this myth of your wife's is a corruption of that?

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    1. That's an interesting one. I don't know how much heat is lost by opening the door or how quickly it recovers. It will, of course, help to do that, but to an extent that matters? Again, thermocouple is needed to test.

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    2. My immediate thought is (a) sticking it up a notch for a moment won't increase the temperature significantly, and (b) opening the door loses heat in the oven extremely rapidly (otherwise you couldn't get your hand in to get the food out).

      Of course if you're opening it to get the food out to serve then you're going to be putting the food in a room temperature environment away from the oven, and its cooling rate will be only on the temperature difference between the food and the room.

      Other side thought.. if the initial increased heat did cause the food to be hotter when you took it out, it would mean it would actually cool down faster..

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  4. Control theory is definitely fun (well, depends on your outlook). It turns out you're both right, more or less. All efficient thermostats have a small proportional zone near the set temperature. The result turns out to be the quickest way to achieve a stable target temperature (when optimally damped). If just getting an empty oven to temperature is the objective then 'setting to 9' slows this down because of the over-shoot and instability, not the rate of blast (but more on this later).
    So what about electric heating (or an old central heating thermostat) where, as you say, the thermostat has a binary output? Well, in this case, you'll usually find the thermostat itself includes a small internal heater which completely overcomes any mechanical hysteresis. In the 'proportional zone' the internal heater, thermostat and mechanical hysteresis create a slow oscillator, making the heating cycle on and off with some duty cycle even if the temperature seen by the thermostat is perfectly stable. The duty cycle then naturally changes with offset from the set temperature to maintain a very stable room temperature. Disconnect the internal heater and the thermostat needs to see bigger changes in temperature before switching the heating but, worse than this, the system becomes inherently unstable resulting in significant over-shoot.
    Back to the oven... the difference in warm-up time is probably negligible either way. However, there's also a more complex experienced-based process in play in that a little bit of over-shoot will partly compensate for the initial cooling caused by introducing a cold dish to cook at the right moment, accelerating the overall objective in a generally desirable manner. Never argue with the cook!

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  5. I've had similar arguments. My car varies the amount of heat so turning up full helps.

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  6. I'd also argue that your wife is right, but for a different reason. For evenly and consistently cooked food, it's important with an oven that the whole oven (rails, shelves, sides, trays etc.) gets up to temperature and not just the air in the oven. The thermostat measures the temperature at just one place in the oven.
    Say that you want a temperature of 160C....
    If the thermostat is set to 160C, then the flames will be cut when the thermostat reaches 160C. This is unlikely to be when the whole oven has reached 160C. The flames being cut, it's going to take a lot longer for the whole oven to reach 160C.
    If the thermostat is set to, say, 190C, then more energy is pumped in once 160C is reached, heating up the rest of the oven. Of course, leave it at 190C for too long and then the whole oven will be too hot.
    Essentially, if you have all the time in the world, set the oven to 160C and wait. If you don't, set it higher and then knock it back.

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  7. I see the same silly behaviour of people with thermostats, e.g. air conditioners, heaters etc. But let me give you some relationship advice: Just agree with your wife whether she is right or not.

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    1. I am sick of people saying you should always agree with the woman whether she is right or not. If women are to be given equal respect from men, they need to stop insisting on winning every argument.

      My sister in law is like this, you are not allowed to disagree with her in "her" house (ignoring my brother's share of it) even if she's completely wrong. We don't get on.

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  8. Hmmmm, so is it better to be correct for the wrong reason or incorrect but backed by a (maybe incorrect) thought process?

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    1. I just wish some times I could have a reasonable debate with some people.

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  9. On the last couple of gas ovens I've had, the temperature control has not been binary like a typical thermostat-driven water boiler - there's a minimum flame used to start, a full flame, and then when the oven temperature is "near" the set point, the flame varies. This can be evidenced by pre-heating to say mark 5, then observing the flame while adjusting the dial and noticing the analogue variation in flame hight and/or sound.

    On my current oven, "near" is approximately +/- 3 gas marks. This means that setting higher will prevent retardation of flame when approaching the desired temperature. It does require constant monitoring or timing though if you don't want to overshoot, but in the case of my current oven, it can reduce the time to heat to mark 6 by over 15 minutes

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  10. It's my understanding that most ovens and home furnaces and A/C units are binary in function ... but their operation is hidden. Is it possible that people are taking their visual experience of a stoves operation (electrical or gas, there's a range of values that can be set) and applying it to ovens, furnaces, and A/C units?

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  11. Have a look into PID loops, you'll find that both of you are both correct and wrong at the same time when it comes to an electric oven. (Can't say about gas ovens or any specific models).
    You will find that an oven will reach its target temperature faster if you set it there first, but it gets hotter faster if you set it to full power first, but it will overshoot and have to come back down and will have bigger fluctuations.

    Control loop theory is quite a big subject but happy to provide graphs etc to explain!

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    1. Things have moved on a lot from simple bi-metalic strip thermostats, yes, but there are plenty of applications where the control is just on or off, and you can't PWN it sensibly, so it is the same control logic.

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